The instrument
The GDM-121 microphone corresponds to the MD-21 from Sennheiser and was manufactured for Grundig from 1959 to 1965.
Details
Dynamic microphone, omnidirectional characteristic. Transmission range 50 to 17,000 Hz, impedance 200 ohms, sensitivity 1.5 mV/Pa. With tripod thread and table stand. Integrated impedance converter, connection cable with 3-pin diode plug.